cathode bypass capacitors in pre3, omitting them?

[bikerhifinut]

I have been musing over this circuit since obtaining a set of parts to build one.

As designed the circuit has a gain of around 15.5dB as I understand it.

So I wondered how much gain would be lost by omitting one, or both, cathode bypass capacitors on the ECC82. I think if my theory is right that this introduces an amount of cathode feedback which cuts gain, without upsetting the DC operating conditions.

Am I barking up a dead end alley here?

I even figured that leaving out a stage of amplification on the pre3 might give the same results, its a zero feedback design as far as I can see so there shouldn't be any complications there.

My power amps are all around 500mV or less sensitivity so unity gain is usually enough, with the odd source I have only outputting 200 mV so a gain of 6db would be plenty in that case. I had a notion to include a simple switch to isolate the bypass cap(s) in order to have adjustable gain.

And just to add complication, I was also pondering the use of something like a 6SN7 instead of ECC82, just because I can but am willing to be persuaded otherwise.

Please no shunt pot suggestions, my feelings on that mod are well known to most of the regulars here. And I feel it treats the symptoms rather than the problem so to speak. I have the desire to use my switched attenuator in this as my set up in the new house allows the preamp and sources close to the listening position with power amp(s) at the far end of the room and very short speaker wires.
A lot of ideas all at once, apologies for that.
Thanks in advance gentlemen.

Crazy Andy.

[John Caswell]

Hi Andy,
I can't see any reason why you cannot do this, not sure how much it will reduce the gain, I suggest you try it and see. Unfortunately my Pre3 is too buried in the system to quickly try it out.
What did do with mine is to fit an intermediate preset volume control 250KΩ -1MΩ in place of L1-L2 and R2 (1MΩ) and set it to a sensible listening level, you could then remove the pot measure it and replace it with two fixed resistors. This does have the advantage of reducing the noise from the first stage.
I also tried removing the first stage completely but that made the whole thing sound pretty lifeless.
When time allows I am going to experiment with feedback a) to see if it is possible and b) to see what effect/it sounds like, but these things are on the back burner at present due to serious family problems.

John

[pre65]

Originally Posted by John Caswell

but these things are on the back burner at present due to serious family problems.

John

Sorry to hear that John, I hope all will be well before too long.

[bikerhifinut]

Originally Posted by pre65

Sorry to hear that John, I hope all will be well before too long.

Me too, John.

I'm investigating these mods too so if I find anything I'll post what I discover.
interesting about the first stage removal as I pondered that too.
I have a sneaky suspicion that removing a bypass cap might impair the bass end? But as its an easy experiment it's worth doing.

I'm now thinking of putting the phono3 board in the same box as I have a large case. (I may buy a new PCB off Matthew for this).

Andy

[John Caswell]

Hi all,
Thank you for your comments, much appreciated.
Andy, (I'm now thinking of putting the phono3 board in the same box as I have a large case. (I may buy a new PCB off Matthew for this)) not a bad idea, Pre3 case is a bit cramped. You should have seen it before I attacked the layout way back when.
My Pre3 is in a 2U case with RF remote volume, relay input switching and a few other bells and whistles.
It enables you to play with the both the SUTs and the Pre3 optxs to minimise hum.

John

[FreddieT]

John,

I hope all turns out well for you and yours. You may remember a while back that you soldered the bypass caps back in a Pre3 that I borrowed from Matthew and (with permission) removed the caps. I can't remember why I didn't do it myself. I used to have a good memory I think.

Andy,

I do remember that the gain was reduced (as theory suggests, you are introducing negative feedback) but can't find my notes (see above) on my measurements. The bass was affected, but it should be less attenuated (the bypass cap sets the LF roll off) relative to the midband. However, the Pre3 caps are quite large and I don't remember significant differences. Incidentally smaller values are often used as treble boosters in guitar amps. Give it a go, and I'll continue to search on and off for my scribblings.

F.

[bikerhifinut]

Just bumping this up.
I am getting the parts together to complete a preamp based more than heavily on the pre4, I accumulated a set of WD parts from various sources, mainly Nigel! and another kind member on here except the remote control bits which are currently being mulled over but on balance its looking like I'll purchase the official control board from here, it seemed expensive but when I count up the cost of parts from elsewhere plus the brain ache of working out how to wire it all in the WD board starts to look pretty good value.
Anyway, I have a notion to build it with Octals, 6SN7, but that I think would mean a necessity for an extra PSU as I believe the PSU3 transformer won't be capable of supplying 2.1A at 6.3V, the heater draw will be 1,2A from 2x6SN7 rather than 600mA from 2 x ECC82. It also rules out 6CG7 on that basis that they draw 600mA each on the heaters. Yeah yeah I know the ECC82 will work as well as anything else in this application, and lets face it they are cheap and plentiful. So I may just go with what is known to work.
So I remember a conversation with John C a wee while back and if I remember rightly I can reduce the gain by splitting R1 into 2 separate resistors and take the coupling capacitor to the 2nd stage off the tap as a potential divider? This was a technique used by mullard in their "Mixing Preamplifier" design and they gave values for various overall gains. Again it should help reduce noise from stage 1?
I'm doing the homework now so that come may/june when the building works done here and I get my workrooom back I can crack straight on and build it.

A.

[bob orbell]

Andy, don't forget that the PSU III powers 5 valves, plus two small relays, your choice of two 6SN7's should be no problem unless you are also asking PSU III to power a phono as well. Bob

[bikerhifinut]

Originally Posted by bob orbell

Andy, don't forget that the PSU III powers 5 valves, plus two small relays, your choice of two 6SN7's should be no problem unless you are also asking PSU III to power a phono as well. Bob

You got it Bob, the transformer I think is rated for 2A for the heaters and I need 2.1A. Ht current aint an issue but LT is. I have a couple of spare toroids and the wherewithal to make an extra PSU so all is not lost if i go down the octal or 6CG7 route. the thing is my regulated PSU board will deliver 3A at 6.3V if the transformer is capable but there it is and looks like i shall knock up a separate PSU. I've got a diecast box here that I can shoehorn a PSU into and I'll just hide it away from the line of sight.

Onwards and upwards

[bikerhifinut]

Of course if I'd looked back up the therad I would have seen Johns instructiions.
Ok so I am looking to get 2 to 3x gain, so far so good.
What would be an optimum oreder of magnitude for the potential divider, i.e to get 3x gain its a PD of equal values on each leg so do I use 2x 470k which kind of lets the anode of V1 see the same 1meg after the coupling capacitor and shouldnt alter the frequency response noticeably?
Or do I reduce it by a factor of 10 and use say 2x47k, this shows the 0.1uF looking into 100k which may be a bit low, bearing in mind , if my calculation is correct theres about 6k output Z off the first stage which suggests to me a minimum of 60k it should be looking into?
The idea I had was that high value resistive loads were potentially noiser than a lower load?
And in the same vein do I increase the Value of C1? as designed its 0.1uF.

A.